Article fabrication from powders



July 7, 1959 w. A. MAXWELL ET AL ARTICLE FABRICATIGN FROM POWDERS Filed Jan. 7. 1954 INVENTORS William/1. Maxwell, liaymond \Sfiarm'ck ATTORNEYS M Allen QF/zzrzcisco a/ZZ, AK W N G G D V R a l R 6 E 0 T M u M ma M Mm m N k E N F R S COOL ING r J \i! WA TER VACUUM VACUUM VACUUM United States Patent ARTICLE FABRICATION FROM POWDERS William A. Maxwell, Bay Village, Raymond S. Gui-nick, Cuyahoga Falls, and Allen C. Francisco, Berea, Ohio, assignors to the United States of America as represented by the Secretary of the Navy Application January 7, 1954, Serial No. 402,828

4 Claims. (Cl. 25-156) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to formation of articles from powders, with particular application to powder metallurgy. In powder metallurgy, for example, articles have been formed by cold pressing and sintering, by hot pressing, or by slip or solid castings. However, with the exception of the casting method considerable machining or grinding is necessary to form the object, and the casting method is not suitable for all applications.

Briefly stated, the present invention is directed to article formation by mixing a small amount of liquid, with or-without a binder, into the powdered refractory material, injecting the material into a mold, freezing the material, drying the molded material so as not to crack or deform the molded shape, and sintering the dried form.

The objects of the invention, therefore, include provision of a method of article formation free of pressing steps, a method permitting ready use on varied and intricate forms, a method eliminating breakage of weak intermediate structures and a method permitting facile use of a continuous manufacturing line.

C Other objects and advantages of this invention will appear on reference to the following description and accompanying drawings in which:

Fig. 1 is a flow diagram indicating the process steps in article formation;

Fig. 2 is a view showing one arrangement for deaerating a powdered charge;

Fig. 3 is a view illustrating the injection of the material into a mold; and

Fig. 4 is a view of a turbine blade made according to the described process.

While the process is applicable to any ordinary powdered material, with refractory materials it has been found particularly useful, such materials as molybdenum disilicide, titanium-carbide, aluminum oxide, and iron, having been found exceptionally suitable. As an example of use the treatment of molybdenum disilicide will be described.

Molybdenum silicide is first granulated to about nine microns average size which results in maximum strength after sintering and then formed in a slip by mixture with a suflicient amount of water to form a thick nonflowing slip. In a typical example 3.55 milliliters of water is added to 20.0 grams of molybdenum disilicide powder. While not always essential it is usually desirable to include a small amount of binder, as starch, in solution in the water, about 0.4 gram to equal about 2% of the weight of the dry powder.

The slip (Fig. 2) is then placed in a chamber forming the injection cylinder 11 of a piston-cylinder unit 12, the piston 13 at the cylinder base being slidable therein. A latch 14 may be provided to hold the pisice ton at extended limit for retention of the entire powder supply.

The piston 13 is attached at its base to a vibrator plate 15 which, by any appropriate means, as by a 60 cycle adjustable amplitude vibrator 16, is oscillated to keep the powder mix in a state of constant motion. At its upper end the cylinder 11 is externally ridged, as at 17, and flattened at the end to form an engagement surface for the vacuum head coupling 18 in the deaeration process step, or for the die unit in the subsequent step of mold injection. Tubing 19 connects the vacuum head to a vacuum pump (not shown).

For the de-aeration step the vacuum head coupling is applied to the contact surface of the injection cylinder, a sealing ring 20 being interposed between these units to prevent leakage. "The vacuum line is then established and the vibrator energized, thus agitating the powdered slip and aiding in gas removal therefrom.

The vacuum head is then separated by the die 30. This die is of the usual two part construction with interior contours fashioned to the desired article form. The die parts are of equal length and are flattened to engage at one end outer end 17 of the injection cylinder and at the other end the vacuum head 18, annular leakage seals 31 and 32 being provided. Applying suction and vibration to the unit, together with light hand pressure, and releasing latch 14, the slip is caused to move into the die mold spaces and pack solidly therein.

Both dies and the contained slip are now placed in a freezing unit such as a solid carbon dioxide bath at 40 (3.; and after ten minutes, or a time dependent on the slip mass, the die is removed from the bath, opened and the casting removed.

It is now necessary to remove the moisture in the casting, which, as previously stated, is just sutlicient to produce a thick non-flowing slip. To accomplish this result without disturbance of the molded form structure the drying takes place at a pressure permitting sublimation, two to four millimeters pressure being found satisfactory for water during a time period of about four hours. Cooling of the drying chamber to hold the green casting below the melting point may be employed. After drying, the mold is sintered at around 2300 C. for one hour in a five micron vacuum. Fig. 4 illustrates a turbine blade 40 made according to the described process.

While the underlying process has been described it is desirable to point out certain useful refinements therein and to indicate desirable uses and materials.

While fine powders can be employed singly, it is usually desirable, as mentioned above, to add a small amount of binder in order to increase the green strength of the molded product to permit handling. Any inert binder may be used which forms a suspension or solution with the admixed liquid, does not impart excessive viscosity to the slip, is not harmed by freezing or solidification and imparts adequate strength to the dry casting when used in quantities of a few percent. Binders meeting these requirements are gelatines or starches, synthetic starches being particularly satisfactory.

In the die structure, a low temperature casting material permitting easy fabrication is desirable such as plaster of Paris, rubber, plastic or metal. Various casting alloys have proved satisfactory.

The particle size of the powder is a factor deserving attention in the process, the used powder usually having a size of optimum effectiveness. For example, with molybdenum disilicide a nine micron average diameter is indicated, with titanium carbide, four microns, and with iron powders, diameters preferably not exceeding about 200 mesh. These particle sizes are selected in order that the finished casting will exhibit maximum strength after sintering. The freeze casting step has the important advantage of permitting a wide range of particle sizes as compared to the conventional slip casting methods wherein the particle sizes depend upon the time allowed for absorption and the required fluidity of the slip. Within limits the larger the particle size the lower the amount of liquid required to make a castable slip. However, the range of size is determined by suitability for the final sintering step rather than absorption time and required fluidity.

Water has been used as a slip agent giving satisfactory results. However, any liquid may be used possessing to some degree the following properties: (a) it should maintain a low viscosity when mixed with the powder to form a slip; (b) it should solidify preferably in the range from room temperature down to the limit of economical refrigeration; (c) it should not react chemically with the powder; (d) it should dry or evaporate from the solid or liquid state either in vacuum or in air; and (e) preferably, it should not be toxic or hazardous in use. For example, cyclo-hexanol has been employed and paraffin hydro-carbons solidifying at room temperature are available for use. As appears from the disclosure, the added liquid maintains the green molded shape when frozen so as to permit handling. On sublimation of the liquid the binder holds the form until the sintering step establishes mass cohesion.

It is important to note that the described process does away with the usual pressing and shaping steps of powder metallurgy. Using molybdenum disilicide and the method of vibratory injection, a green shaped object is produced having a density equal to that produced by conventional pressing at 10,000 pounds per square inch. By causing the slip to flow under vibration, the amount of liquid required can be decreased and, thus, the resulting green density increased in comparison with the liquid quantities required and the densities attained in conventional slip castings. Further, the use of a viscous, non-flowing slip lessens the possibility of segregation of the refractory powders during casting.

An important use of the process is in the field of infiltration. In conventional infiltration techniques ceramals are produced by infiltering various molten metals or alloys into a porous skeleton frame, thereby making possible the reduction of strategically critical metals and an increase in operating temperatures. These prior methods involve die pressing of the skeleton powders sufficiently to produce the desired porosity, as to 40%. This skeleton may be infiltered directly or semisintered prior to infiltration.

According to the freeze-cast method a skeleton mate rial, such as titanium carbide, in powdered form, is cast as previously described in a shaped mold having a porosity of about 40%. The casting is then semi-sintered to obtain particle bonding, the porosity being reduced to about Infiltration is then carried on using molten Inconel or other high temperature alloy, the capillary action of the porous skeleton indrawing the filler metal to form a solid form. The usual procedure is to place the infiltrant in physical contact with the skeleton material preferably in an evacuated chamber and inductively heat the infiltrant above the point of fusion. (See US. Patent No. 2,456,779.) The advantage of the freeze-cast method as applied to infiltration is apparent in that various intricate shapes not practical with the usual pressure methods may readily be employed.

Modifications of the invention are possible in the light of the above teachings; and hence the invention may be practiced otherwise than as specifically described, within the scope of the appended claims.

What is claimed is:

1. A process of forming articles from a refractory powder which consists in mixing a liquid with said powder to form a thick, non-flowing slip, moving the slip into a mold while subjecting the slip to vibration, freezing the slip to form a casting, removing the mold, drying the casting in a vacuous atmosphere at a temperature below the melting point of the liquid and sintering the casting.

2. The process of forming articles from solid refractory material which consists in forming the material into a powder having an average diameter of about nine microns, mixing water with the material in the proportion of 3.55 milliliters to 20 grams of the powder to form a thick, non-flowing slip, injecting the slip into a mold by imparting vibration and suction thereto, freezing the slip to form a casting, removing the mold, drying the casting in a vacuous atmosphere at a pressure less than about four microns and a temperature below the freezing temperature of the liquid, and sintering the casting at a temperature around 2300 C.

3. The process of making articles as defined in claim 2 with the initial mix of powdered refractory material and water including a starch binder in the amount to form two percent of the dry powder-starch mix.

4. The process of forming articles from a refractory material of one of the group consisting of molybdenum disilicide, titanium carbide and aluminum oxide comprising the steps of forming the material into a powder having a particle size of optimum effectiveness during sintering, mixing a liquid with the powder to form a thick, non-flowing slip, deaerating the slip, injecting the deaerated slip into a mold by imparting vibration thereto, freezing the slip to form a casting, removing the mold, drying the casting by sublimation, and sintering the casting.

References Cited in the file of this patent UNITED STATES PATENTS 1,694,563 Ross et al. Dec. 11, 1928 2,196,258 Erdle Apr. 9, 1940 2,444,124 Wedler June 29, 1948 2,456,779 Goetzel Dec. 21, 1948 2,476,726 Haas July 19, 1949 2,622,304 Coffer Dec. 23, 1952 2,645,836 Sorensen et al. July 21, 1953 2,669,762 Blackburn et al. Feb. 23, 1954 2,765,512 Nesbit Oct. 9, 1956 

1. A PROCESS OF FORMING ARTICLES FROM A REFRACTORY POWDER WHICH CONSISTS IN MIXING A LIQUID WITH SAID POWDER TO FORM A THICK, NON-FLOWING SLIP, MOVING THE SLIP INTO A MOLD WHILE SUBJECTING THE SLIP TO VIBRATION, FREEZING THE SLIP TO FORM A CASTING, REMOVING THE MOLD, DRYING THE CASTING IN A VACUOUS ATMOSPHERE AT A TEMPERATURE BELOW THE MELTING POINT OF THE LIQUID AND SINTERING THE CASTING. 